This invention relates generally to commercial deep fat fryers, and more particularly relates to computer controlled, high powered, electrically heated deep fat frying systems utilizing solid state switching devices.
Current deep fat fryers used by commercial establishments for cooking large quantities of french fries and similar products are typically powered by three phase 240 volts. All such systems use mechanical or mercury type contactors to control the high current to the heating elements, often in a duty cycle modulated manner for temperature control. Each heating element usually includes a first set of contactors controlled by a high limit thermostat and associated circuitry which is connected in series with the primary contactors and the heating elements. The primary contactors are switched on and off by a thermostat control circuitry in order to achieve the precise temperature control required to produce consistent food products of acceptable quality. In this procedure, the primary contactors must be cycled on and off repeatedly. But, in order to keep the total switching cycles over the expected lifetime of the contactors to a reasonably low number, the contactors can only be switched at a relatively low rate. Duty cycles on the order of 30 seconds are typical. Such duty cycle periods are often greater than the thermal inertia of the heating elements resulting in temperature variations at the surface of the elements in contact with the cooking oil.
The use of triacs, a semiconductor switching device, as the main switching device for controlling power to a heating element has been suggested by prior art devices as disclosed in U.S. Pat. No. 3,946,200, to Juodikis, issued Mar. 23, 1976. This patent discloses the use of a triac controlled by a bridge circuit and zero voltage switches to provide switching of the triac at zero cross over points to achieve proportional control of the power to the heating element and thus a constant temperature.
Commercial deep fat frying units of a type used in fast food establishments for cooking large loads of french fries and the like, typically employ a pair of separate cooking vats within each unit, and may have a number of different units. In these systems, the cooking oil degrades rapidly when subjected to excessive temperatures, and the cooking oil, together with power, are among the higher cost items of the business. In the normal operation of previous systems, the relatively long duration of the duty cycles result in the elements being left on for a number of seconds, followed by an off cycle during which the element cools down. As a result, the oil is exposed to peak temperatures greater than would be necessary if a more uniform temperature of the heating elements could be maintained.
These types of cooking systems have in recent times employed computers to control the on/off cycle of the cooking elements in a manner to minimize the maximum temperature of the heating elements during the melt cycle when solid shortening is first being melted and may not be in contact with portions of the heating elements. Even if liquid shortening is used, during the morning heat up cycle, the oil must be preheated to the set point or other standby temperature. The oil is typically heated rapidly with full power since there are typically a number of separate frypots, the power consumption of the facility exceeds the peak power defined by the local utility which often results in increased rates per unit of power.